1. Field of the Invention
The present invention relates to an optical amplifier. More particularly, the present invention relates to a gain-clamped semiconductor optical amplifier having a horizontal lasing structure, in which an oscillation direction of a laser for gain-clamping is different than an amplification direction of a signal, and a method for manufacturing the gain-clamped semiconductor optical amplifier.
2. Description of the Related Art
In a general optical communication system, when light that is emitted from a transmitter moves along an optical transmission line, it suffers transmission losses, whereby a signal arriving at a receiver becomes reduced. When the power of light arriving at a receiver is smaller than a predetermined value, normal optical communication may be not performed due to a receiving error. Therefore, an optical amplifier is provided between a transmitter and a receiver so as to amplify light, thereby compensating for the transmission loss of the light transmitted along the optical transmission line and enabling the light to be transmitted to a farther distance with little error.
Such optical amplifiers include an erbium-doped fiber amplifier (hereinafter, referred to as EDFA), a Raman amplifier, and a semiconductor optical amplifier (hereinafter, referred to as SOA).
The EDFA, which uses an optical fiber doped with the rare-earth elements (e.g. an Erbium) for amplification, has a high gain characteristic, a low noise figure (NF), and high saturation output power, so that the EDFA has been widely used in both a backbone network and in a metro network. However, the EDFA has problems in that the price is high and an operation wavelength is limited.
A Raman amplifier, which uses Raman amplification in an optical fiber, is a method for amplifying light using a so-called Raman amplification phenomenon. According to Raman amplification, when the pumping of a strong light is incident to the optical fiber, a gain appears at a longer wavelength side distanced about 100 nm from wavelength of the pumping light due to stimulated Raman scattering. Light of the wavelength band having above gain is incident to the excited optical fiber, so that light is amplified. The Raman amplifier can easily adjust an amplification band by properly setting wavelength of the pumping light for the Raman amplification, and has a low noise figure. However, the Raman amplifier has disadvantages in that not only does it have very low optical amplification efficiency but also needs a high-priced pumping light source, thereby increasing the entire size of the optical amplifier module and the price of the optical amplifier module.
The SOA uses gain characteristics of a semiconductor and can adjust its amplification band according to a semiconductor band gap. The SOA has advantages in that it is small in size (usually a few cm) and especially does not require a high-priced pumping light source.
However, the SOA generally has a gain saturation phenomenon at low input power, and that a gain value decreases when intensity of an input signal increases. Therefore, when a signal having a large optical power is inputted, the inputted signal is distorted during signal amplification to be transmitted.
In order to solve such a problem, a gain-clamped SOA having a structure as shown in FIG. 1 has been proposed.
FIG. 1 illustrates a structure of a conventional gain-clamped semiconductor optical amplifier (gain-clamped SOA) 100. The gain-clamped SOA 100 includes an n-InP substrate 101, an InGaAsP passive waveguide layer 102, an InP spacer 103, a DBR lattice pattern 104, an active-layer waveguide 105, a current blocking layer 106, a p-InP clad layer 107, a p-InGaAs layer 108 for reducing an ohmic contact resistance, an oxide layer 109, an upper electrode 110, and a lower electrode 111.
The gain-clamped SOA 100 induces laser oscillation in a short wavelength that is outside of a wavelength range of an input signal to be amplified. The gain-clamped SOA uses distributed Bragg reflector lattices to fix the density of carriers in a resonator, so that optical gain is constantly maintained even though a driving current increases.
However, in the conventional gain-clamped SOA, one problem exists in that a first procession direction (shown as “A” in FIG. 1) of a signal is the same as a second procession direction (shown as “B” in FIG. 1) of a laser beam to induce oscillation. Therefore, when signals of several channels are amplified, a four wave mixing phenomenon is caused between the oscillation wavelength and a signal wavelength. Further, the conventional gain-clamped SOA has another problem in that a separate wavelength filter is required for removing the oscillation wavelength of the laser.